1999 GMC Sierra 1500 numbers are not matching

[99SierraBthebest]

Hello, I have a 99 GMC Sierra 1500 SLT Z 71 off-road package.

There is a huge backstory to this truck that makes it very important to me. 
throughout the two years I’ve had it I’ve been upgrading and building it. 

every time I go to the parts store it seems like there is an issue with the Vin number and the parts they give me. 

Believe me I know the incompetence of most part stores . 

NAPA auto parts seems to be the only parts store that has knowledgeable

older employees competent enough to do an actual Vin search with cross referencing. They were able to use the cast numbers on my rear differential to verify what I had already thought. 
The rear end is out of an 05 GMC Yukon XL.  
all of the work that I have done has always been a chore first because the 99 is a 2 Truck year. 
there is a old body style 99 and a new body style 99

while at Napa they let me actually look at their computer screen and I noticed when They enter
I started looking at Vin number identification and cross-referencing with RPO codes. 
The Vin number States that its 5.3 L V8 

The RPO code says it’s a 4.3 L 

on the fan shroud it says 5.3 L V8 

I look at my chilltom  book and it tells me to look at the motor identification placard
Saying that it is at the base of the back of the block where the transmission bell housing mounts. With a picture showing exact location. 
there is no placard there. 
so I check the cast numbers on the rear of the block passenger side. 

This is the biggest thing I’m having problems with.  I ran those numbers on four different websites. 
on two of those websites There was no match through the search  I checked small block and big block cast numbers from 1978 till 2013. 
these numbers do not match to anything 

Chevy put out. 
everyone keeps telling me it’s a small block 5.3 but the numbers start with 1256 

those starting numbers do appear in the big block section. 
so I looked through it and found two  Big block motors with almost exact numbers but off by one or two .

 

I was starting to think that this is looking like a chop shop ordeal. 
when replacing the transmission and transfer case I noticed the wiring harness was mostly zip tied and not connected to stock anchor points. 

also the truck has 386,000 miles on it. 

The body is beat to crap.
But the motor is exceptionally strong with exceptional power. 
My brother has an 05 Silverado with the same fifth wheel tow package same Z 71 heavy suspension and 5.3 L it was very real well taken care of by an old man the only used it to tow his camper. His truck only had 90,000 miles and my sierra out performs his in every single way. 
it has much more torque much more get up and go. and can haul so much more.

with little to no problem on any incline

he keeps telling me he doesn’t think it’s a 53 because of the power of the motor and the mileage

The fender skirts had been ripped off on the front causing  old mud splatter all throughout the engine compartment. But the motor is surprisingly clean except for a few valve cover leaks.  
it’s so clean it almost looks out of place. 

The coil packs  are not new but they have no mud splatter but the plug wires have mud all over them. 
I could go on for days with reasons why I think the motor has been swapped. 

because of not being able to verify block casting numbers. 
I cannot be sure of what parts to buy or know what known issues I’m dealing with. 

I have read that 99 to 03 has some issues with PCV valve and badly engineered driver-side valve cover. 

intake gasket failure. 
I and Cass tech heads that crack causing head gasket failures. 

as soon as the weather changed warm the symptoms of all of these issues came up immediately. 

I am no mechanic and this is my first  NBS Chevy. I am very mechanically inclined but I don’t understand electronics computers and vacuum in these new trucks.  
if anyone can give me any information on The Block cast #12560392  

with lettering NMP6GMPT. 

or give me any verifiable information on how I can decipher exactly what motor this is what heads those are. 
I would like to start with the service bulletin pertaining to changing the driver   Side valve cover gasket with better baffle and integrated PCV valve.

feel free to ask any pertinent information you need. 
I tried to be thorough sorry for the small buck

Thank you and God bless

 

 

 

[CamGTP]

The part number #12560392  is an oil pan number for a rear sump on a 4.8/5.3/6.0 engine.

 

The RPO code for a 5.3 is LM7 and LR4 for a 4.8, the tag is in the glove box which I'm sure you've found. If you had a bore scope you could figure out what engine it was too. The 5.3's use a dished piston and the 4.8's use a flat top piston.

 

The cylinder heads will also have one of these numbers on them, 706 / 862 / 895.

 

Looks like your passenger side head says 862 and I think the 862's where from 1999-2000 and the 706's started in 2001 but that could be off. So I'd say it's a good bet that it is a 5.3 truck.